1. Field of the Invention
The present invention relates to an electrode forming method a bump electrode formable base used therefor, and more particularly to an electrode forming method for forming a bump electrode and a bump electrode formable base used in the production method.
2. Description of the Background Art
In accordance with rapid development of internet networks and spread of portable information devices to markets, higher speed and higher functions as well as scale reduction and weight reduction are sought for in semiconductor packages. In order to meet these demands, the modes of semiconductor packages are changing rapidly from peripheral terminal type packages, such QFP (Quad Flatpack Package) to area array type packages, such as BGA (Ball Grid array) and CSP (Chip Scale Package).
In accordance with such change in the modes of semiconductor packages, what is attracting people""s attention for establishing connection between a semiconductor device and a circuit substrate is a flip-chip connection which can connect pins at a high density (multi-pin connection) and which is excellent in electrical characteristics as well.
The flip-chip connection is a method in which electrodes (bump electrodes) made of a low-melting-point metal alloy are formed on a semiconductor package, and the bump electrodes are brought into contact with a predetermined place of a substrate or the like to establish an electrical and mechanical connection between the semiconductor package and the substrate by heat fusion. In order to provide such a flip-chip connection, various methods of forming a bump electrode have been developed.
As a method for forming such a bump electrode, the solder paste printing method and the plating method may be mentioned. By adopting these methods, a solder material can be collectively supplied onto pad electrodes, thereby giving a high productivity. However, these methods require cleaning and removal of flux components for activating the solder surface, which are used in a thermal treatment step for melting the supplied solder material.
As a means for solving such problems of the solder printing method and others, Japanese Patent Laying-Open No. 62-257750/1987 proposes a method of forming pad electrodes with the use of a dispenser nozzle. This method is a method in which a molten solder is jetted from the dispenser nozzle for fixation onto the pad electrode.
An underlayer metal of chromium, copper, and gold is laminated on the surface of the pad electrode in order to ensure wettability. Further, the pad electrode is heated to at least 100xc2x0 C. A suitable amount of solder grains are jetted from the tip of the dispenser nozzle towards the pad electrode by introducing a gas such as nitrogen into the dispenser nozzle and applying a pulse-like pressure to the molten solder by operating a gas on-off valve. By this process, a bump electrode is formed on the pad electrode.
However, according to the aforesaid method of using a dispenser nozzle, a sufficient joining strength between the bump electrode and the pad electrode may not be obtained depending on the surface condition or the temperature of the pad electrode. As a result, it may not be possible to obtain good electrical connection between the semiconductor package and the substrate.
The present invention has been made in order to solve the aforementioned problems of the prior art. One object of the present invention is to provide an electrode forming method capable of ensuring a joining strength between the bump electrode and the pad electrode. The other object of the present invention is to provide a bump electrode formable base used for such an electrode forming method.
The first one of the electrode forming method according to one aspect of the present invention is a method for forming a bump electrode on an underlying conductive region, and includes an electrode forming step for ejecting a molten solder for adhesion onto the underlying conductive region that has been set at a temperature of at least 60xc2x0 C. and lower than a melting point of the solder.
It has been found out that, according to this electrode forming method, by setting the temperature of the underlying conductive region to be at least 60xc2x0 C. and lower than the melting point of the solder, an alloy layer is formed between the solder and the underlying conductive region to provide a sufficient joining strength between the bump electrode and the underlying conductive region. It has also been found out that, if the temperature of the underlying conductive region is lower than 60xc2x0 C., this alloy layer is not formed. On the other hand, it has been found out that, if the temperature of the underlying conductive region is higher than or equal to the melting point of the solder, wrinkles are formed on the surface of the adhering solder, thereby producing a bump with a distorted shape.
In view of enhancing the cleanness of the surface of the underlying conductive region, the electrode forming step preferably includes a step of performing plasma-deaning on the surface of the underlying conductive region by exposing the underlying conductive region to a plasma atmosphere in advance.
In this case, the wettability of the underlying conductive region is enhanced and, even if the temperature of the underlying conductive region is comparatively low, a sufficient alloy layer is formed between the solder and the underlying conductive region, so that the joining strength between the two is further improved.
Furthermore, if the cleanness of the surface of the underlying conductive region is not so high and the wettability is comparatively low, the electrode forming step preferably includes a step of setting the temperature of the underlying conductive region to be at least 150xc2x0 C.
In this case, even if the wettability of the underlying conductive region is low, a sufficient alloy layer is formed between the solder and the underlying conductive region, so that the joining strength between the two can be enhanced.
Further, the electrode forming step preferably includes a step of performing rapid cooling and solidification on the underlying conductive region and the solder after adhesion of the molten solder onto the underlying conductive region.
In this case, by rapid cooling and solidification, crystal grains of the molten solder and the alloy layer formed between the solder and the underlying conductive region become dense, so that the joining strength between the two can be further enhanced.
Further, the electrode forming step preferably includes a step of applying supersonic wave to the underlying conductive region in allowing the molten solder to adhere onto the underlying conductive region.
In this case, the oxide coat present on the surface of the molten solder is easily broken by the supersonic wave when the molten solder impinges upon the underlying conductive region. Therefore, the surface of the underlying conductive region is wetted by the solder, and the solder spreads easily over the entire surface to form an alloy layer sufficiently between the solder and the underlying conductive region. As a result, the joining strength between the bump electrode and the underlying conductive region can be further enhanced.
The second one of the electrode forming method according to one aspect of the present invention is an electrode forming method for forming a bump electrode on an underlying conductive region, and includes a step of forming a projection on a surface of the underlying conductive region and ejecting a molten solder towards the projection for adhesion.
According to this electrode forming method, the oxide coat present on the surface of the molten solder is easily broken by the projection when the molten solder impinges upon the underlying conductive region. Therefore, the surface of the underlying conductive region is wetted by the solder, and the solder spreads easily over the entire surface to form an alloy layer sufficiently between the solder and the underlying conductive region. As a result, the joining strength between the bump electrode and the underlying conductive region can be improved.
In view of enhancing the cleanness of the surface of the underlying conductive region, the electrode forming step preferably includes a step of performing plasma-cleaning on the surface of the underlying conductive region by exposing the underlying conductive region to a plasma atmosphere after forming the projection and before ejecting the molten solder.
In this case, the wettability of the underlying conductive region is enhanced, and the joining strength between the bump electrode and the underlying conductive region is further improved.
Further, the electrode forming step preferably includes a step of applying supersonic wave to the underlying conductive region in ejecting the molten solder towards the underlying conductive region.
In this case, the oxide coat present on the surface of the molten solder is easily broken by the supersonic wave when the molten solder impinges upon the underlying conductive region. Therefore, the surface of the underlying conductive region is wetted by the solder, and the solder spreads easily over the entire surface to form an alloy layer sufficiently between the solder and the underlying conductive region. As a result, the joining strength between the bump electrode and the underlying conductive region can be further enhanced.
A bump electrode formable base according to the other aspect of the present invention is a bump electrode formable base in which a bump electrode is to be formed on an underlying conductive region by ejecting a molten solder towards the underlying conductive region, wherein the bump electrode formable base includes a projection formed in the underlying conductive region for breaking an oxide coat present on a surface of the molten solder when the molten solder impinges upon the underlying conductive region.
According to this bump electrode formable base, the oxide coat present on the surface of the molten solder is easily broken by the projection when the molten solder impinges upon the underlying conductive region. Therefore, the surface of the underlying conductive region is wetted by the solder, and the solder spreads easily over the entire surface of the underlying conductive region. This allows an alloy layer to be sufficiently formed between the solder and the underlying conductive region. As a result, the joining strength between the bump electrode and the underlying conductive region is improved.
Specifically, it is preferable that the underlying conductive region is formed on a semiconductor chip or is formed on a substrate.
This enhances the joining strength between the semiconductor chip and the bump electrode or the joining strength between the substrate and the bump electrode.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.